In X-ray imaging, for example, in computed tomography, angiography or radiography, counting direct-conversion X-ray detectors or integrating indirect-conversion X-ray detectors can be used.
The X-ray radiation or the photons can be converted in direct-conversion X-ray detectors into electric pulses by a suitable converter material. As a converter material, for example CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs or other substances can be used. The incident X-ray radiation deposits energy in the converter material, whereby electron-hole pairs are formed. By the application of a potential difference, in particular a high voltage of approximately −1000 V, electrons and holes are caused to drift to the opposing electrodes. Electric pulses are passed on to the evaluating unit. The electric pulses are evaluated by an evaluating electronics system, for example, an integrated circuit (Application Specific Integrated Circuit, ASIC).
In counting X-ray detectors, incident X-ray radiation is measured by counting the electric pulses that are triggered by the absorption of X-ray photons in the converter material. The size of the electric pulse is typically proportional to the energy of the absorbed X-ray photon. By this, an item of spectral information can be extracted through the comparison of the size of the electric pulse with a threshold value.
From the document DE 10 2015 201 494 A1, there is known a method for determining the polarization state of a sensor of an X-ray detector, the X-ray detector being illuminated with a light pulse sequence. The individual pulses of the light pulse sequence have a different intensity. It is further determined at what intensity of the light pulses, charge pulses generated by the sensor of the X-ray detector exceed a pre-set threshold voltage of a signal detection circuit.
From the document DE 10 2012 213 410 B3, there is known a direct-converting X-ray detector for detecting X-ray radiation which comprises at least one semiconductor used for the detection of X-ray radiation and at least one electrode applied to the semiconductor. The semiconductor and the at least one electrode are electrically conductively connected, the at least one electrode being transparent and configured electrically conductive.